1. Field of the Invention
The present invention relates to an image forming apparatus of a copier, a printer, a facsimile or a hybrid machine of these or the like and a fixing device used therefor.
2. Description of Related Art
Conventionally, an on-demand type fixing device with a short rise time is widely known in an image forming apparatus of a copier, a printer or the like (for example, refer to JP2002-6656A).
The on-demand type fixing device is constituted from a fixing film (fixing member such as an endless film), a pressure roller (pressure member), a heater such as a ceramic heater or the like (heating device) and so on. The heater is set up in an internal part of the fixing film, comes into contact with the pressure roller via the fixing film to form a nip part and heats the fixing film. Then a toner image on a recording media delivered towards the nip part receives heat and pressure in the nip part and is fixed to the recording media.
On the other hand, JPH11-133800A, JP2000-162907A and JPH5-53461A disclose a technology that disposes a plurality of heaters (heating device) in an on-demand type fixing device for the purpose of preventing a flaw in which the fixing member heat deteriorates because both end parts of the fixing member have excessive temperature rise when small sized paper is consecutively fed. Specifically, in correspondence to a size in a width direction of a recording media delivered to the nip part, a plurality of heaters are disposed parallel or in line against the width direction so that the area to be heated in the width direction against the fixing film (fixing member) can be set variable.
The heating device such as the heater or the like of the above described conventional on-demand type fixing device has negative replacement property (maintenance property). This is described in detail hereinbelow.
Because the life span of a heater is finite, maintenance and replacement of the heater is performed frequently in the fixing device. However, because the heater is pressure-welded to the pressure roller via the fixing film, it is difficult to pull out the heater applied with pressure directly in the width direction (longitudinal direction).
In order to solve such a flaw, a method of disposing a mechanism that cancels out the pressure between the heater (fixing film) and the pressure roller is also considered. That is, by operating the pressure cancellation mechanism, the heater can be pulled out from the device after the pressure between the heater and the pressure roller is cancelled out. However, in that case, extra cost and space are necessary for the set up of the pressure cancellation mechanism.
In particular, in a conventional on-demand type fixing device, pressure by the pressure member is always applied against the heater. A constitution is adopted in which the heater is easily subject to breakage during jam processing and delivery or the like so that the above described problems cannot be neglected.
In addition, the on-demand type fixing device in the above described JPH11-133800A, JP2000-162907A and JPH5-53461A or the like includes a plurality of heaters disposed so that the above described problems are especially not negligible. In addition, in the case the sizes of a plurality of heaters differ, a nip width of the nip part formed by pressure-welding the heater and the pressure roller becomes non-uniform across the width direction so that fixture irregularities are generated on output images.